Problem: Which of these numbers is prime? ${2,\ 48,\ 91,\ 93,\ 99}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 2 are 1 and 2. The factors of 48 are 1, 2, 3, 4, 6, 8, 12, 16, 24, and 48. The factors of 91 are 1, 7, 13, and 91. The factors of 93 are 1, 3, 31, and 93. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 2 is a prime number.